Both butter milk and butter serum are pale yellow liquids which are generated as by-products together with the butter and butter oil of interest in producing butter from cream prepared to a milk fat content of from 30 to 40% in the former case, and in producing butter oil from butter or high fat cream in the latter case, by a physical fractionation operation such as centrifugation which makes use of the difference in specific gravity or demulsification represented by churning effected by the collision of milk fat globules. By further concentrating or drying (butter milk powder) these liquids, various types of butter milk-related dairy products and processed milk products are produced. In Europe and America, butter milk is also produced when fermented butter is produced, after fermenting cream by inoculating a lactic acid bacterium, and this is generally called acidic butter milk. On the other hand, a product by a process without fermentation is called sweet butter milk. Objects of the present invention do not distinguish kinds of sweet and acidic butter milks. Also, though butter milk and butter serum are distinguished based on the difference of the dairy products used in their production, basic differences are not found in terms of their milk components, physical properties and the like, so that butter serum is regarded in the present invention as a part of butter milk in a broad sense (hereinafter, the term butter milk sometimes includes butter serum).
It is said that butter milk contains lipids, proteins, carbohydrates, minerals, vitamins and the like similar to the case of milk, and its nutritive value per solid matter is almost the same as that of milk. However, as can be understood from its production method, it is characterized by the presence of a large amount of milk fat globule membrane substance constituting the interface of milk fat globules, and it is known that milk fat globule membrane substance-characteristic lipoproteins and glycoproteins are present as the membrane proteins, in addition to casein, α-lactalbumin and β-lactoglobulin. Also, it is said that it contains a large amount of phospholipids which are generally referred to as lecithin, in addition to the general triglycerides (neutral lipids) as the membrane constituting lipids, occupying from 29 to 36% of the milk fat globule membrane substance. The main phospholipids are phosphatidylcholine, sphingomyelin and phosphatidylethanolamine, and when compared with lecithin derived from a plant such as soybean, the milk lecithin is particularly characterized by the presence of sphingomyelin in a considerably large amount of approximately 18%, which is close to the composition of human milk lecithin, so that its nutritional application value is considered to be markedly high.
In addition, since butter milk richly contains aromatic components which are characteristic to butter, it is a desirable material from the viewpoint of flavor, too. The aromatic components are composed of ketones, fatty acids, lactones and the like, and are particularly characterized in that ketones are contained in an overwhelmingly large amount of 78% even when compared with other dairy products.
On the other hand, it is said that butter milk is also effective for the treatment of acute hepatitis and gastric mucosal sepsis and the prevention of diarrhea of babies. It is also considered that it has a blood pressure reducing activity and serum cholesterol reducing activity.
Thus, in spite of the excellent characteristics such as high nutritive value, various physiological functions and reinforcement of milk flavor of food, the application of butter milk as a food material has hardly been developed, because it is apt to undergo oxidation due to the dissolved oxygen from the air which quickly generates unpleasant taste and smell. Currently in Japan, butter milk generated during the production of butter is mostly discarded without any application, which must be regarded as a serious loss of a food resource.
Two methods are known for effecting long-term storage of butter milk, namely (1) a method in which the number of microorganisms in the generated butter milk is controlled to a predetermined level or less by a batch or plate heating treatment, and the resulting butter milk is concentrated under a reduced pressure using an evaporator, subjected to final sterilization and then frozen to obtain a butter milk concentrate and (2) a method in which the butter milk concentrate obtained in (1) by concentrating under a reduced pressure using an evaporator is made into powder using a spray drying apparatus (dryer). Since butter milk is highly concentrated in either method, the produced butter milk-related diary products are apt to undergo oxidation by oxygen in the air, considerably strongly, so that in order to avoid this, each product is generally packed in a can or an aluminum foil-lined polyethylene processed paper container and stored in a cool and dark place. However, even when it is stored in a cool and dark place using these sealed containers, foul taste and smell due to generation of oxidized products becomes strong within approximately 1 to 3 months, or within about 1 week after opening of the container, so that the shelf life is considerably poor and its application value as a food processing material (e.g., “Nyuseihin Kogyo (Dairy Products Industry), the second volume”, published on Feb. 25, 1972, pp. 223-224, edited by Tomokichi Tsugo, published by Chikyu Shuppan) is low.
In addition, by taking note of the fact that butter milk contains milk fat globule membrane substance in a large amount, it has been reported on a method in which the fat globule membrane substance is fractionated and purified from butter milk and used as a food flavor improving agent (JP-A-7-236451) and a method in which the milk fat globules are removed from butter milk by centrifugation and then the ultrafiltration vitrification liquid is used as a food flavor improving agent (Japanese Patent No. 3004912). However, each of these methods is a technique related to the fractionation and purification of the milk fat globule membrane substance in butter milk, and it is evident that this technique is basically different from the technique of the present invention which is intended to prevent generation of oxidized smell caused by undergoing influence of the oxidation based on the dissolution of air oxygen in butter milk, thereby keeping and improving the milky flavor inherent to the butter milk. In addition, although the degree of flavor may vary, as will be described later, generation of oxidized smell of the flavor improving agents obtained by these techniques cannot be avoided, so long as a heat treatment is carried out in the presence of unsaturated fatty acids and milk proteins.
In the field of dairy industry, a process has been reported in which a drink having a flavor close to crude milk or unheated solution of milk is produced by carrying out heat treatment of milk or a milk-containing unheated solution after reducing dissolved oxygen in the liquid prior to their heat treatment (JP-A-10-295341), and also a sterilization method which does not generate heated smell, as a method for sterilizing milk and the like by replacing dissolved oxygen therein with nitrogen gas, in which sterilization is carried out after reducing amount of dissolved oxygen in milk and the like by replacing dissolved oxygen by nitrogen gas, effected by the combined use of a means wherein nitrogen gas is directly mixed and dispersed in milk and the like and a means wherein milk and the like which are not contaminated with nitrogen gas are sprayed from the upper side through a nozzle to nitrogen gas-mixed and dispersed milk and the like stored in a nitrogen gas-replaced tank under a nitrogen gas atmosphere (Japanese Patent No. 3091752). However, the objects of these methods for keeping and improving flavor are milks of cow, horse, goat, sheep, water buffalo, human and the like mammals, and products containing these milks, such as milk based drinks, processed milk, fortified or separated milk beverages, reduced milk, fermented milk, lactic acid bacteria beverage, fresh cream, fruit juice drink and the like, but nothing is reported therein about the butter milk which undergoes influence of the oxidation by oxygen in the air considerably strongly as described in the foregoing.